The present application is related to co-pending U.S. patent application titled xe2x80x9cSystem and Method for Writing to a Magnetic Shift Register,xe2x80x9d which was filed on even date herewith, which is assigned to the same assignee as the present application, and which is incorporated herein by reference.
The present invention generally relates to memory storage systems, and particularly to a memory storage system that uses the magnetic moment of magnetic domains to store data. Specifically, the present invention relates to a system that uses current to move magnetic domains across read and write devices, allowing data to be stored in a shiftable magnetic shift register.
The two conventional common non-volatile data storage devices are: disk drives and solid state random access memories (RAM). Disk drives are capable of inexpensively storing large amounts of data, i.e., greater than 100 GB. However, disk drives are inherently unreliable. A hard drive includes a fixed read/write head and a moving media upon which data is written. Devices with moving parts tend to wear out and fail. Solid state random access memories currently store data on the order of 1 GB (gigabyte) per device, and are relatively expensive, per storage unit, i.e., per 1 GB, compared to a disk drive.
The most common type of solid state RAM is Flash memory. Flash memory relies on a thin layer of polysilicon that is disposed in oxide below a transistor""s on-off control gate. This layer of polysilicon is a floating gate, isolated by the silicon from the control gate and the transistor channel. Flash memory is relatively slow, with reading or writing times on the order of a microsecond. In addition, flash memory cells could begin to lose data after less than a million write cycles. While this may be adequate for some applications, flash memory cells would begin to fail rapidly if used constantly to write new data, such as in a computer""s main memory. Further, the access time for flash memory is much too long for computer applications.
Another form of RAM is the ferroelectric RAM, or FRAM. FRAM stores data based on the direction that ferroelectric domains point. FRAM has access times much faster than Flash memory and consumes less energy than standard dynamic random access memory (DRAM). However, commercially available memory capacities are currently low, on the order of 0.25 MB (megabyte). In addition, memory storage in a FRAM relies on physically moving atoms, leading to eventual degradation of the medium and failure of the memory.
Yet another form of RAM is the Ovonic Unified Memory (OUM), which utilizes a material that alternates between crystalline and amorphous phases to store data. The material used in this application is a chalcogenide alloy. After the chalcogenide alloy experiences a heating and cooling cycle, it could be programmed to accept one of two stable phases: polycrystalline or amorphous.
The variation in resistance of the two phases leads to the use of the chalcogenide alloy as memory storage. Data access time is on the order of 50 ns. However, the size of these memories is still small, on the order of 4 MB currently. In addition, OUM relies on physically changing a material from crystalline to amorphous; which likely causes the material to eventually degrade and fail.
Semiconductor magnetoresistive RAM (MRAM) stores data as direction of magnetic moment in a ferromagnetic material. Atoms in ferromagnetic materials respond to external magnetic fields, aligning their magnetic moments to the direction of the applied magnetic field. When the field is removed, the atoms magnetic moments still remain aligned in the induced direction. A field applied in the opposite direction causes the atoms to realign themselves with the new direction. Typically, the magnetic moments of the atoms within a volume of the ferromagnetic material are aligned parallel to one another by a magnetic exchange interaction. These atoms then respond together, largely as one macro-magnetic moment, or magnetic domain, to the external magnetic field
One approach to MRAM uses a magnetic tunneling junction as the memory cell. The magnetic tunneling junction comprises two layers of ferromagnetic material separated by a thin insulating material. The direction of the magnetic domains is fixed in one layer. In the second layer, the domain direction is allowed to move in response to an applied field. Consequently, the direction of the domains in the second layer can either be parallel or opposite to the first layer, allowing the storage of data in the form of ones and zeros. However, currently available MRAM can only store up to 1 Mb (megabit), much less than needed for most memory applications. Larger memories are currently in development. In addition, each MRAM memory cell stores only one bit of data, thereby limiting the maximum possible memory capacity of such devices.
What is therefore needed is a memory device that may bridge the gap between the low cost and high capacity but fundamentally unreliable mechanical disk drives, and the high cost and, by comparison with disk drives, much lower capacity, of solid state RAMs. This memory should have a comparable capacity to that of disk drives, at competitive prices, but advantageously does not use moving parts, and does not require physical state changes to the material. The need for such a system has heretofore remained unsatisfied.
The present invention satisfies this need, and presents a system and an associated method (collectively referred to herein as xe2x80x9cthe systemxe2x80x9d or xe2x80x9cthe present systemxe2x80x9d) for a magnetic shift register, writing device, and reading device. Briefly, the present system uses the inherent, natural properties of the domain walls in ferromagnetic materials to store data. The present system utilizes one read/write device to access numerous bits, on the order of 100 bits of data or more. Consequently, a small number of logic elements can access hundreds of bits of data.
The present system uses spin based electronics to write and read data in ferromagnetic material so that the physical nature of the material is unchanged in the magnetic shift register of the present invention. In one embodiment, a shiftable magnetic shift register comprises a data track formed of a fine wire or strip of material made of ferromagnetic material. The wire may be comprised of a physically uniform, magnetically homogeneous ferromagnetic material or layers of different ferromagnetic materials. Information is stored as direction of magnetic moment within the domains in the track. The wire can be magnetized in small sections in one direction or another. An electric current is applied to the track to move the magnetic domains, along the track, in the direction of the electric current, past a reading or writing elements or devices. In a magnetic material with domain walls, a current passed across the domain wall moves the domain wall in the direction of the current flow. As the current passes through a domain, it becomes xe2x80x9cspin polarizedxe2x80x9d. When this spin polarized current passes into the next domain across a domain wall, it develops a spin torque. This spin torque moves the domain wall. Domain wall velocities can be very high, on the order of 100 m/sec.
In summary, a current passed through the track with a series of magnetic domains with alternating directions, can move these domains past the reading and writing elements. The reading device can then read the direction of the magnetic moments. The writing device can change the direction of the magnetic moments, thus writing information to the track.
According to a preferred embodiment of the present invention, one read/write element is dedicated to a single track, with the understanding that in other embodiments, more than one read and/or write elements could be assigned to one or more tracks.
Associated with each domain wall are large magnetic fringing fields. The domain wall concentrates the change in magnetism from one direction to another in a very small space. Depending on the nature of the domain wall, very large dipolar fringing fields can emanate from the domain wall. This characteristic of magnetic domains is used to write to the magnetic shift register. When the domain wall is moved close to another magnetic material, the large fields of the domain wall change the direction of the magnetic moment in the magnetic material, effectively xe2x80x9cwritingxe2x80x9d to the magnetic material.
An important characteristic of domain wall fringing fields is that they are localized in small regions of space near the domain wall. Thus, domain wall fringing fields can provide highly localized and large magnetic fields that can be manipulated in space by moving or controlling the position of the domain wall within a magnetic entity such as a magnetic wire. Since the magnitude of the fringing fields drops rapidly with distance from the domain wall, application of the domain wall fringing fields can be controlled in wires by varying the distance of the wire from the material, whose property is to be changed by the domain wall fringing field and by moving the domain wall along the wire.
This concept for using fringing fields to write to a magnetic material can be applied to many different applications using spintronics, including but not limited to: magnetic random access memories; magnetic recording hard disk drives; magnetic logic devices; security cards using magnetically stored information; semiconductor devices wherein large magnetic fields provided by domain wall fringing fields can be used to locally vary the electronic properties of the semiconductor or semiconductor heterostructure; mesoscopic devices, which are sufficiently small that the electronic energy levels, therein, can be substantially affected by the application of local magnetic fields; and so forth.
For applications involving the manipulation of spin-polarized current in semiconductors, spin-polarized current is injected into a semiconductor or semiconductor heterostructure, and then is manipulated as desired, according to the specific application in which the present invention is used. If a very large local magnetic field is applied to the semiconductor, certain electronic levels in the semiconductor can be spin-split, changing the electronic state of the semiconductor. For example, the electronic state of the semiconductor can be changed from being conductive to being non-conductive. Consequently, the use of a device such as the fringing field write device can be used to switch a semiconductor from xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d.
The influence of a magnetic field on a semiconductor or semiconductor heterostructure is determined, in large part, by the gyromagnetic ratio, g-factor, of the semiconductor. The larger the g-factor, the larger is the affect of the magnetic field on the electronic properties of the semiconductor. So, it may be advantageous, to use the present system in conjunction with semiconducting materials with large g-factors.
Reading the data on the magnetic shift register can be accomplished, for example, using standard technology such as a magnetic tunneling junction. A magnetic tunneling junction has two magnetic materials separated by a very thin insulating layer, or tunneling barrier. The magnitude of any current passed through the tunneling barrier depends on the relative magnetic orientation of the two magnetic materials in the tunneling junction. Consequently, the value of the current in the tunneling junction indicates the direction of the magnetic moment in the magnetic shift register that is being read. For further details about the design and performance of the magnetic tunneling junction and the exchange biased magnetic tunnel junction, reference is made to U.S. Pat. Nos. 5,650,958; 5,729,410; and 5,801,984, that are incorporated herein by reference.
By incorporating the magnetic shift register as part of the magnetic tunneling junction, information stored in the domains in the magnetic shift register could be read by the current that passes through the magnetic tunnel junction. As the domains flow pass the magnetic tunneling junction, the magnitude of the current indicates the value stored by the direction of the domain. Moving the domains around the magnetic shift register brings the chosen domain to the magnetic tunneling junction for reading purposes.
The magnetic shift register described herein, presents numerous advantages over other forms of solid state memory and magnetic recording hard disk drives. In particular, the magnetic shift register provides a means of accessing hundreds of data bits using a small number of logic gates and circuit elements. Thus, the magnetic shift register can provide capacious amounts of storage comparable to those provided in conventional hard disk drives but without any moving parts and at a comparable cost of such hard disk drives.
Similarly, compared to conventional solid state memory devices, the magnetic shift register provides far higher memory capacities but at a fraction of the cost per bit of conventional solid state memories. This advantage is achieved because the magnetic shift register can be fabricated using standard CMOS processes and methods of manufacture but the magnetic shift register stores hundreds of data bits for the same area of silicon in which a conventional CMOS solid state memory would store one or a small number of bits.
This latter advantage is realized because the magnetic shift register uses the third dimension out of the plane of the silicon substrate to store data in largely vertical tracks which occupy little space on the silicon substrate. Since the cost of CMOS logic and memory is largely determined by the area of silicon used in any given technology node the magnetic shift register can thereby provide a far cheaper means of storing data than conventional solid state memories.
The magnetic shift register can thus be used to replace many existing data storage devices, including but not limited to magnetic recording hard disk drives, and many solid state memories such as DRAM, SRAM, FeRAM, MRAM, etc.
The capacity of the magnetic shift register can be varied over a wide range continuously by simply varying the number of magnetic shift register tracks per memory device. This is a particular advantage over magnetic hard disk drives in which because of the high cost of the reading and writing heads and their circuitry, and the high cost of the mechanical means of moving these heads and the magnetic media, a hard disk drive only provides a cheap means of storage when many gigabytes of data are stored, such that the cost of the mechanical components of the hard disk drive is amortized over the large number of data bits.
By contrast, the magnetic shift register can be built at low cost per bit in much smaller sizes, thereby allowing the magnetic shift register to be used for a wide range of applications where the data storage capacity required is much lower than that of a magnetic hard disk drive. Thus the magnetic shift register can be used for various electronic devices including by way of example, but not limited to: digital cameras, personal digital assistants, security devices, memory sticks, removable storage devices, and so forth.